1. Field of the Invention
The present invention relates to a semi-conductor manufacturing process. More particularly, it relates to a method for protecting stepper alignment marks.
2. Description of the Prior Art
Alignment marks are indispensable for the photolithographic processing of semiconductor wafers. Alignment marks help ensure that the masks which are necessary for exposing the photoresist on the semiconductor wafer can be aligned exactly. This presupposes that the alignment marks have an exact assignment to the structures to be fabricated on the semiconductor wafer and, furthermore, offer a sufficient topographical contrast to be able to be reliably identified.
For power IC products, damage to the scribe line due to metal etching process-related problems usually cannot be avoided. However, along the scribe, there are many stepper alignment marks, which are used for stepper alignment while exposing the wafer. Due to this damage, these marks cannot be used for alignment causing difficulties with the photo process.
In order to ensure thorough etching in the metal etching process, when a portion requiring lower etching rates is finished, an underlying film beneath an earlier completed etching portion will also be etched, resulting in over-etching. Since the thickness of the metal layer in power IC products is thicker (3 xcexcm) than general products, under normal process circumstances, the scribe line will be damaged after the metal etching process. The marks cannot be used for alignment on subsequent photolithographic processes.
FIG. 1 is a diagrammatic top view of a substrate after etching a metal layer thereon. The substrate 10 is divided into the integrated circuit area 12 and the scribe line 14, and the alignment mark 16 is situated on the scribe line 14. As can be seen, the scribe line is damaged after the metal etching process, thus rendering the alignment marks indistinct. The consequence is an increased outlay on post processing on account of the mask""s misalignment. Alignment may even become impossible. This problem is becoming more prevalent as the result of ever-decreasing structure widths and increased demands for alignment accuracy.
Accordingly, an object of the present invention is to provide a method for protecting stepper alignment marks. Since process-related damage only occurs after metal etching, in order to protect all the alignment marks during the metal etching process, a protection pattern (mark) is added right above the mark needed to be protected on the metal photo layer mask. By doing this, the photoresist remains after the metal photo layer and the photoresist is used to protect all the alignment marks from the metal etching. After metal dry etching, the photoresist remaining on top of the alignment marks is removed and the undamaged marks underneath can be used for stepper alignment.
According to the present invention, a method for protecting stepper alignment marks suitable for a substrate with an alignment mark on a scribe line and a metal layer that will be etched includes the following steps. First, a photoresist layer is formed over the metal layer. Next, a photo mask which has a predefined photo mask pattern for transfer to the metal layer is provided, and a pattern protecting the alignment mark is added to the photo mask pattern. Then, a photolithographic process is performed with the photo mask pattern on the photoresist layer to form the desired transferring photoresist mask to the metal layer and the protective photoresist mask for protecting the alignment marks.